Hi Regal,
the higher the frequencies and faster rise times the moreanalogue problems you have, the main one being the higher harmonics.
Yes a true square wave is an impossibility, and these days with DDR memory interfaces, the waves when viewed on a scope look more like sharkfins than a square wave.
Intrestingly, with the ever increasing embedded processing involved with even the most mundane electronic equipement these days, one has to wonder how much if any effect firmware could have on audio quality A simple thing would be FPGA and similar based designs, lowering the rise times and thus switching currents can help with the overall system noise floor.
One of the problems with consumer based digital gear is cost of manufacture, quite often compramisers are made (especialy with layout, mainly layer counts), these often add to system noise. One would hope that high end digital equipement has proper multilayer boards (12-14), which is what PC's should have as a minimum, a lot dont though.
As to bluetooth, its becoming popular for remote controls, that should add a bit more noise to the EMC spectrum, off course if you use power line communications you'll have plenty of noise around before you even plug in anything else.
I look at5 the elctrical noise like light polution, when I was young I could see the stars, and if I pluged myself into a scope, I could see a 50Hz sine, now If I lkook at the sky I can see a couple of stars, and when I plug myself into a scope...noise.
My favorite time to listen to music these days is late at night, when all are in bed, no network, no TV,s, little background noise both electrical and airborne, bliss. Whether it affect the sound I cant say, or measure, I havn't the kit, but mentally it always sounds better to me.

Thankyou, digital design is really fascinatingly complex a 12-14 layer PCB is mind blowing to me ! I am heading back to the tube forum